Systems and methods for removal of mercury and/or hydrochloric acid from gas streams using calcium-containing particles

ABSTRACT

Several embodiments of the present technology are directed to the removal of one or more air pollutants using cooling and/or calcium-containing particles. In some embodiments, a method for removing air pollutants comprises flowing a gas stream having calcium-containing particles and one or more of mercury or hydrochloric acid molecules, and cooling the gas stream, thereby causing at least a portion of the calcium-containing particles to adsorb to the mercury and/or hydrochloric acid molecules in the gas stream. The method can further comprise, after cooling the gas stream, filtering the gas stream to remove at least a portion of the calcium-containing particles having adsorbed mercury and hydrochloric acid.

TECHNICAL FIELD

The present technology is generally directed to the removal of airpollutants from gas streams using calcium-containing particles. Inparticular embodiments, the present technology relates to the removal ofmercury and/or hydrochloric acid from gas streams using cooling andcalcium-containing particles.

BACKGROUND

Exposure to high levels of mercury, hydrochloric acid and otherhazardous air pollutants are associated with neurological anddevelopmental disorders in human beings. As such, the discharge of theseair pollutants from industrial facilities is limited and regulated bystate and federal jurisdictions. Conventional methods for abating theseair pollutants today include injecting powder activated carbon (PAC)into the exhaust gas stream containing the air pollutants. The PAC canbind to the air pollutants and be filtered from the exhaust gas streambefore it is discharged to atmosphere. The use and/or injection of PAC,though, can be a generally dirty process, as carbon dust associated withPAC can coat and foul downstream equipment, and be difficult to clean.Furthermore, the costs associated with removing and/or disposing usedPAC is an expensive process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block flow diagram of an industrial processingsystem in accordance with embodiments of the present technology.

FIG. 2 is a schematic flow diagram of a method for removing airpollutants from a gas stream in accordance with embodiments of thepresent technology.

FIG. 3 is a schematic flow chart of a method for operating a system usedto remove air pollutants from a gas stream in accordance withembodiments of the present technology.

FIG. 4 is a schematic flow diagram of a method for removing airpollutants from a gas stream in accordance with embodiments of thepresent technology.

DETAILED DESCRIPTION

The present technology is generally directed to the removal of airpollutants from gas streams using calcium-containing particles. In someembodiments, the present technology includes the removal of mercury (Hg)and/or hydrochloric acid (HCl) from gas streams via cooling andadsorption via calcium-containing particles. As described in more detailbelow, a method for removing one or more air pollutants from a gasstream can include flowing a gas stream at a first temperature toward anexhaust stack, and then cooling the gas stream to a second temperature.The gas stream can include calcium-containing particles (e.g., calciumoxide (CaO)), and air pollutants including mercury (e.g., elementalmercury (Hg⁰) and/or oxidized mercury (Hg²⁺)) and/or hydrochloric acid.At the second temperature, the calcium-containing particles have anincreased adsorptive affinity for the mercury and/or hydrochloric acidpresent in the gas stream relative to the first temperature. The methodfurther comprises adsorbing at least a portion of the mercury and/orhydrochloric acid to the calcium-containing particles, and filtering thegas stream to remove at least some of the calcium-containing particleshaving the adsorbed mercury and/or hydrochloric acid.

Numerous specific details are set forth in the following description andfigures to provide a thorough and enabling description of embodiments ofthe invention. One skilled in the relevant art, however, will recognizethat the invention can be practiced without one or more of the specificdetails. In other instances, well-known structures or operations are notshown or are not described in detail to avoid obscuring aspects of theinvention. In general, alternatives and alternate embodiments describedherein are substantially similar to the previously describedembodiments, and common elements are identified by the same referencenumbers.

FIG. 1 is a schematic block flow diagram of an industrial processingsystem 100 configured in accordance with embodiments of the presenttechnology. As shown in the illustrated embodiment, the system 100receives a gas stream (e.g., a flue gas, exhaust stream, process stream,etc.) at an inlet 102 from an upstream industrial process and directsthe gas stream toward a discharge point 104 (e.g., an exhaust stack)where the gas stream is discharged from the system 100. The system 100can include a controller 106, a temperature adjustment zone 108 thatreceives at least a portion of the gas stream from the inlet 102, abaghouse 110 positioned downstream of the temperature adjustment zone108, a fan 112 (e.g., an induced draft fan) positioned downstream of thebaghouse 110, one or more injection systems 114 positioned upstream ofthe baghouse 100, and a plurality of sensors. The temperature adjustmentzone 108, baghouse 110, fan 112, injection system(s) 114 and pluralityof sensors can be operably coupled to the controller 106 such that theycan be individually monitored and/or individually controlled by thecontroller 106. As shown in the illustrated embodiment, the plurality ofsensors can include a first temperature sensor 120 positioned upstreamof the temperature adjustment zone 108, a second temperature sensor 122positioned downstream of the temperature adjustment zone 108, adifferential-pressure sensor 124 monitoring pressure drop of thebaghouse 110, a flow sensor 126 positioned downstream of the fan 112,and one or more air pollutant sensors 128 downstream of the baghouse110. The air pollutant sensor(s) 128 can be part of a continuousemissions monitoring system (CEMS), and be used to measure aconcentration of mercury (e.g., total mercury, oxidized mercury, and/ornon-oxidized mercury), hydrochloric acid, nitrogen oxides (NO_(x)),sulfur oxides (SO_(x)), and/or other regulated hazardous air pollutants.

The industrial process or industrial apparatus providing the gas streamupstream of the inlet 102 can include that of a lime kiln or cementprocessing facility. In such embodiments, crushed limestone rock isadded to a lime kiln and heated using a fuel source such as coal, coalcoke, natural gas, amongst other combustible materials. The heatingprocess calcines limestone rock into lime, resulting in a flue gasstream that is exhausted by the lime kiln. In other embodiments, theindustrial process can include that of a refinery, paper mill, powerplant or chemical production facility. The gas stream received at theinlet 102 can include oxygen, various hydrocarbons, carbon dioxide,water, carbon monoxide, nitrogen oxides, sulfur oxides, mercury,hydrochloric acid, and other air pollutants, as well as one or morecalcium-containing materials. The calcium-containing materials caninclude a plurality of calcium oxide (CaO) particles, which may also bereferred to as lime kiln dust (LKD).

The temperature adjustment zone 108 can be a region wherein the thermalproperties of the gas stream are adjusted, and include one or more of acooling system or apparatus, heat exchanger, spray cooler, injection ofa cooled (e.g., colder) stream, or other apparatus known in the art foradjusting thermal properties of the incoming inlet gas stream. The heatexchanger can use air as a cooling fluid to remove heat from the inletgas stream. In some embodiments, the inlet gas stream entering thetemperature adjustment zone 108 is greater than about 450°, and the gasstream exiting the temperature adjustment zone 108 is less than about350° F. For example, the gas stream exiting the temperature adjustmentzone 108 can range from about 200° F. to about 350° F., from about 250°F. to about 325° F. from about 275° F. to about 325° F., or from about290° F. to about 310° F., amongst other ranges. The temperatureadjustment zone 108 and mechanisms used to adjust the thermal propertiesof the gas stream (e.g., the heat exchanger) can be operably coupled tothe controller 106, which can control the temperature of the gas streamexiting the temperature adjustment zone 108 (e.g., adjusting the amountof air being directed to the heat exchanger).

Cooling the inlet gas stream can increase the adsorptive affinity of thecalcium-containing particles for mercury and/or hydrochloric acid in thegas stream. Stated differently, the ability for calcium-containingparticles to adsorb to mercury and/or hydrochloric acid moleculespresent in the gas stream has been shown to increase at coolertemperatures. Without being bound by theory, mercury and/or hydrochloricacid molecules, as well as other air pollutants present in the gasstream, adsorb to the surface of the calcium-containing particles viaphysisorption (e.g., van der Waals forces) and/or chemisorption (e.g.,via covalent or ionic bonding). In some embodiments, mercury and/orhydrochloric acid molecules of the gas stream react with the calciumoxide to form a salt. The salt and/or other LKD-adsorbed pollutants canbe removed from the gas stream via the baghouse 110 or other filteringmeans, as described below. Moisture content of the calcium-containingparticles may also aid the absorption and removal of mercury and/orhydrochloric acid molecules, as well as other air pollutants (e.g.,sulfur dioxide) present in the gas stream.

As noted above, the system 100 can include one or more injection systems114 (only one injection system is shown in FIG. 1) positioned upstreamof the baghouse 110, and configured to inject an injectate into the gasstream. The injection system 114 can include a hopper 140 holding theinjectate, and one or more valves 142 operably coupled to the hopper 140and used to regulate the amount of injectate dispersed into the gasstream. The valve(s) 142 can be electrically connected to the controller106 such that the controller 106 controls the opening/closing of thevalve 142 depending on process conditions. The injectate can includecalcium-containing particles, such as calcium hydroxide particles, whichmay also be referred to as lime hydrate. The injectate can furtherinclude calcium oxide particles, or powder activated carbon (PAC), asdescribed in more detail below. The lime hydrate particles can typicallyhave a diameter ranging from about 4 microns to about 50 microns, and amoisture content less than about 2%, less than about 1%, or less thanabout 0.5%. In some embodiments, the lime hydrate can have a surfacearea greater than about 15 m²/g, greater than about 25 m²/g, or within arange from about 10 m²/g to about 50 m²/g, from about 15 m²/g to about30 m²/g, or from about 20 m²/g to about 25 m²/g. Such lime hydrate maybe referred to as enhanced lime hydrate. The injectate can be used toadsorb to mercury and/or hydrochloric acid molecules present in the gasstream. Similar to the description provided above, the mercury and/orhydrochloric acid molecules of the gas stream react with the calciumhydroxide, which can be removed from the stream of exhaust gas via thebaghouse 110 or other filtering means. Notably, the injectate used toadsorb mercury and/or hydrochloric acid can be in addition to or in lieuof the calcium-containing particles that may be present in the inlet gasstream.

Dispersion of the injectate can cause the calcium-containing particlesto contact the mercury and/or hydrochloric acid molecules of the gasstream. As such, in some embodiments, the injection system 114 may use apressurized gas (e.g., compressed air) or some other source of force tocause the injectate particles to be dispersed into the ductingcontaining the gas stream. To enhance dispersion of the injectate acrossa surface area of the ducting, and thereby potentially increase theamount of mercury and/or hydrochloric acid molecules adsorbed by theinjectate, the injectate may include a combination or coarse and fineparticles that, as a whole, have beneficial flowability characteristics,with the coarse particles having greater momentum and traveling furtherdistances, and the finer particles having higher surface area to volumeratios and reactivity.

As noted above, the system 100 may include multiple injection systems114. In some embodiments, the injectate of the injection system 114 caninclude PAC. The PAC is configured to bind to mercury, hydrochloric acidand other air pollutants, and is removed from the gas stream via thebaghouse 110. In some embodiments, the PAC can be treated with bromine.In embodiments including a PAC injection system, the PAC injectionsystem can be used in tandem with other injection system(s) 114 usingcalcium-containing particles as the injectate, as previously described.Operational considerations for injecting PAC are described in furtherdetail below with reference to FIG. 3.

As shown in the illustrated embodiment, the baghouse 110 is positioneddownstream of the injection system 114 and the temperature adjustmentzone 104. The baghouse 110 is a filtering device used to remove solidparticulate, such as calcium-containing particles, from the gas stream.The filtered gas stream exits the baghouse 110 and the particulatematter remains trapped in the baghouse 110. As shown in the illustratedembodiment, a differential pressure (DP) sensor 124 can monitor pressuredrop across the baghouse 110 or a portion of the baghouse 110, and beused by the controller 106 to determine when to “clean” the baghouse 110to remove trapped particular matter therefrom. For example, when thedifferential pressure across the baghouse increases above apredetermined threshold, the controller can cause one or morecompartments of the baghouse 110 to be blown down by opening acorresponding valve 148 and driving off any particulate matter to ahopper 146. The particular matter in the hopper 146 can then be furtherprocessed for disposal.

The fan 112 (e.g., an induced draft fan) is positioned downstream of thebaghouse 110 and is configured to provide the driving force for movingthe gas stream through the system 100 from the inlet 102 to thedischarge point 104. The fan 112 can be operably coupled to thecontroller 106 and be controlled based at least in part on pressureand/or flow rate of the gas stream in the ducting of the system 100.

The air pollutant sensor(s) 128 can include one or more analyzers formonitoring mercury and/or hydrochloric acid, as well as other airpollutants such as nitrogen oxides, sulfur oxides, etc. As shown in theillustrated embodiment, the air pollutant sensor(s) 128 are positioneddownstream of the baghouse 110 to ensure the analyzed gas stream is“clean” and generally free of particulate matter. The air pollutantanalyzer(s) 128 can be in electrical communication with the controller106. As described in more detail below, measurements from the airpollutant analyzer(s) 128 can be used by the controller 106 to controloperation of the system 100.

As described above, the controller 106 receives inputs from multiplecomponents of the system 100, and can adjust parameters based on thoseinputs. The controller 106 can take the form of computer-executableinstructions, including routines executed by a programmable computer.The controller 106 may, for example, also include a combination ofsupervisory control and data acquisition (SCADA) systems, distributedcontrol systems (DCS), programmable logic controllers (PLC), controldevices, and processors configured to process computer-executableinstructions. Those skilled in the relevant art will appreciate that thetechnology can be practiced on computer systems other than thosedescribed herein. The technology can be embodied in a special-purposecomputer or data processor that is specifically programmed, configuredor constructed to perform one or more of the computer-executableinstructions described below. Accordingly, the term “controller” asgenerally used herein can refer to any data processor. Informationhandled by the controller 106 can be presented at any suitable displaymedium, including a CRT display or LCD. The present technology can alsobe practiced in distributed environments, where tasks or modules areperformed by remote processing devices that are linked through acommunications network. In a distributed computing environment, programmodules or subroutines may be located in local and remote memory storagedevices. Aspects of the technology described below may be stored ordistributed on computer-readable media, including magnetic or opticallyreadable or removable computer disks, as well as distributedelectronically over networks. Data structures and transmissions of dataparticular to aspects of the technology are also encompassed within thescope of particular embodiments of the disclosed technology.

As described in more detail below, the controller 106 can be used tooperate the system 100 based at least in part on maintaining theconcentration of one or more of the air pollutants being discharged tothe atmosphere at or below a predetermined threshold. For example, ifmercury is determined by the air pollutant sensor(s) 128 to be above apredetermined threshold, the controller 106 can cause the temperature ofthe gas stream exiting the temperature adjustment zone 108 to bedecreased, thereby adsorbing more mercury to the calcium-containingparticles in the gas stream and decreasing the amount of mercury presentin the gas stream downstream of the baghouse 110. In addition to or inlieu of the foregoing, if mercury is determined by the air pollutantsensor(s) 128 to be above a predetermined threshold, the controller 106can increase the amount of injectate injected into the gas stream,causing more of the mercury to be adsorbed by the injectate and therebydecreasing the amount of mercury present in the gas stream downstream ofthe baghouse 110. As described above, the injectate can includecalcium-containing particles (e.g., calcium hydroxide) from a firstinjection system and/or PAC from a second injection system. In someembodiments, injective injectate to decrease mercury and/or other airpollutants is only used after first attempting to decrease mercuryand/or other air pollutants via cooling of the gas stream.

An advantage of at least some embodiments of the present technology isthe ability to remove mercury, hydrochloric acid and/or other airpollutants using only cooling of the gas stream, or cooling of the gasstream and the calcium-containing particles of the gas stream. Stateddifferently, an advantage of embodiments of the present technology isthe ability to remove air pollutants from the gas stream without needingto inject additional injectate, such as PAC, into the gas stream. Inconventional methods that remove air pollutants from gas streams ofindustrial facilities, PAC is injected into the gas stream to bind tothe air pollutants, and is then filtered by a baghouse. Injecting PAC,though, is a generally dirty process, as the carbon dust of PAC can coatdownstream structures and processes, and be difficult to clean.Embodiments of the present technology provide other means to remove airpollutants from the gas stream, thereby allowing operators to omit PACfrom the regulator operation of a facility. Another related advantage ofembodiments of the present technology is the decreased costs associatedwith using calcium-containing particles. PAC, for example, can besignificantly more expensive than lime hydrate, often costing 20 timesmore than calcium oxide (e.g., LKD) or calcium hydroxide (e.g., limehydrate) on a weight-by-weight comparison. Furthermore, incorporatingPAC injection into a system can require a separate baghouse andadditional electrical costs associated with operation.

FIG. 2 is a schematic flow diagram of a method 200 for removing airpollutants from a gas stream in accordance with embodiments of thepresent technology. Many of the features referred to in FIG. 2 aredescribed with reference to FIG. 1. As shown in the illustratedembodiment of FIG. 2, the method 200 includes providing a gas streamincluding calcium-containing particles and at least one of mercury orhydrochloric acid (process portion 202). The gas stream can be providedfrom an upstream process such as a lime kiln. The calcium-containingparticles can be included in the gas stream as part of an upstreamprocess (e.g., originating from the lime kiln), or can be injected intothe gas stream, e.g., via the injection system 114. The method 200further comprises cooling the gas stream (process portion 204). In someembodiments, cooling the gas stream can include cooling the gas streamfrom the inlet temperature (e.g., at or above 400° F.) to less thanabout 325° F., less than about 300° F., less than about 275° F., orwithin a range from about 240° F. to about 325° F., from about 250° F.to about 300° F., or from about 260° F. to about 290° F. In someembodiments, temperatures below 250° F., depending on the particularindustrial process, can cause undesirable consequences to occur in thesystem 100 (FIG. 1). For example, at temperature below 250° F., moisturecan condense onto the baghouse 110 and/or undesirable reactions, such asthe production of sulfuric acid (H₂SO₄) can develop. Cooling the gasstream has been shown to increase the adsorptive affinity of thecalcium-containing molecules to mercury and hydrochloric acid. As such,cooling the gas stream can cause more of the mercury and/or hydrochloricacid molecules to be adsorbed to the calcium-containing molecules.Furthermore, cooling the gas stream can be controlled based on ameasured amount mercury and/or hydrochloric acid sensed downstream ofthe baghouse 100. For example, if the measured amount of mercury and/orhydrochloric acid is above a predetermined threshold, the controller 106may further cool the gas stream via the temperature adjustment zone 108in an attempt to adsorb more of the mercury and/or hydrochloric acid tothe calcium-containing particles present in the gas stream.

As shown in the illustrated embodiment, the method 200 can furthercomprise adsorbing a portion of the cooled mercury and/or hydrochloricacid molecules to the calcium-containing particles (process portion206). Adsorption of the mercury and/or hydrochloric acid molecules tothe calcium-containing particles can occur by bringing the mercuryand/or hydrochloric acid molecules in contact with thecalcium-containing particles, which can occur by the natural flow of thegas stream through the system. As such, adsorbing the mercury and/orhydrochloric acid molecules to the calcium-containing particles need notbe an active step that is performed separately from just operating thefacility, but rather may occur as a result of the general flow of thegas stream through the system 100 (FIG. 1). In some embodiments,however, adsorbing the mercury and/or hydrochloric acid molecules to thecalcium-containing particles can be an active step. For example,adsorbing the mercury and/or hydrochloric acid molecules to thecalcium-containing particles can include increasing the turbulence(e.g., the Reynolds number) of the gas stream in an attempt to increasethe contact between the mercury and/or hydrochloric acid molecules andthe calcium-containing particles in the gas stream.

The method 200 further comprises filtering the gas stream to remove atleast a portion of the calcium-containing particles having the adsorbedmercury and/or hydrochloric acid (process portion 208). In someembodiments, filtering the gas stream can be done via the baghouse 110(FIG. 1) and/or other filtering means, such as an electrostaticprecipitator.

FIG. 3 is a schematic flow chart of a method 300 for operating a system(e.g., a trim system) used to remove air pollutants from a gas stream inaccordance with embodiments of the present technology. As shown in theillustrated embodiments, process portions 302, 304, 306 and 308 includethe process portions generally similar to and described with respect toFIG. 2. For example, process portion 302 includes providing a gas streamincluding calcium-containing particles and an air pollutant, processportion 304 includes cooling the gas stream, thereby increasing theadsorptive affinity of the calcium-containing particles for molecules ofthe air pollutant, process portion 306 includes adsorbing a portion ofthe air pollutant to the calcium-containing particles, and processportion 308 includes filtering the gas stream to remove at least aportion of the calcium-containing particles having the adsorbed airpollutant.

As shown in the illustrated embodiment, the method 300 further includesmeasuring the amount of air pollutant molecules (e.g., in parts permillion) present in the filtered gas stream (process portion 310).Measuring the amount of air pollutant is done via an analyzer (e.g., theair pollutant sensor(s) 128) or CEMS configured to detect one or more ofmercury, hydrochloric acid, sulfur oxides, and/or nitrogen oxides. Asdescribed above, the analyzer can be in electrical communication with acontroller (e.g., the controller 106). If the measured amount of airpollutant present in the filtered gas stream is at or below apredetermined threshold, then the controller may not make anyadjustments to the system. The predetermined threshold can be based, forexample, on regulated state (e.g., local) or federal discharge limits.For example, the predetermined threshold for mercury may be less than 50lbs Hg/MM ton lime, less than 30 lbs Hg/MM ton lime, or less than 25 lbsHg/MM ton lime, and the predetermined threshold for hydrochloric acidmay be less than 10 ppm(v), less than 5 ppm(v), or less than 3 ppm(v).If the measured amount of air pollutant present in the filtered gasstream is above the predetermined threshold, then the method may makeadjustments to the system in an attempt to decrease the amount of airpollutant being discharged. Stated differently, if the measured amountof air pollutant present in the filtered gas stream is above thepredetermined threshold, then the system can make adjustments to causemore of the air pollutant molecules to be adsorbed by an injectate.

As shown in the illustrated embodiment, for example, if the amount ofremaining air pollutant is above a predetermined threshold, the method300 can proceed to process portion 314 wherein calcium-containingparticles are injected into the gas stream in an attempt to adsorbadditional air pollutant molecules. Injecting calcium-containingparticles can be done via the injection system 114 described withreference to FIG. 1. As described above, the injected calcium-containingparticles can include lime hydrate, enhanced lime hydrate and/or LKD.The amount (e.g., lb/hr) of calcium-containing particles injected intothe gas stream can be based on the measured amount of the air pollutant.In some embodiments, injecting calcium-containing particles is done onlyafter first attempting to adsorb additional air pollutant molecules viacooling, as previously described.

If after injecting the calcium-containing particles into the gas stream,the measured amount of air pollutant remains above the predeterminedthreshold, the method can make further adjustments to the system todecrease the amount of air pollutant being discharged. As shown in theillustrated embodiment, the method 300 can proceed to process portion318, in which PAC is injected into the gas stream. Injecting PAC intothe gas stream can be done via an injection system, as described abovewith reference to FIG. 1. The amount of PAC injected into the gas streamcan be based on the measured amount of the air pollutant. For example,the amount of injected PAC may be continuously increased (e.g., in astep-wise manner) until the measured amount of the air pollutantdecreases to be at or below the predetermined threshold. After injectingPAC, the method can revert to process portion 312. In a preferredembodiment, the method 300 attempts to decrease the amount of airpollutant by first injecting calcium-containing particles into the gas,and only injects PAC if the injected calcium-containing particles do notdecrease the concentration of air pollutant to below the predeterminedthreshold. This order of preferred operation is based in part on theprice of PAC relative to the calcium-containing particles, and thedifficulty with removing PAC, relative to calcium-containing particles,from the system.

FIG. 4 is a schematic flow diagram of a method 400 for removing airpollutants from a gas stream in accordance with embodiments of thepresent technology. As shown in the illustrated embodiment, processportions 402, 404, 406, 408 of the method 400 are similar to the processportions described above. For example, process portion 402 includesproviding a gas stream including a first plurality of calcium-containingparticles (e.g., particles including calcium oxide) and at least one ofmercury or hydrochloric acid, process portion 404 includes injecting asecond plurality of calcium-containing particles (e.g., particlesincluding calcium hydroxide) into the gas stream, process portion 406includes adsorbing a portion of the first and second pluralities ofcalcium-containing particles to the mercury and hydrochloric acid, andprocess portion 408 includes filtering the gas stream to remove at leastsome of the first and second pluralities of calcium-containing particleshaving adsorbed mercury or hydrochloric acid. Notably, method 400differs from method 200 described above in that method 400 does notinclude cooling of the gas stream. As such, cooling the gas stream canbe done in addition to process portions 402, 404, 406, 408, but may notbe necessary in some embodiments.

Examples

Multiple tests were conducted to evaluate and verify the presenttechnology, as has been described above. In some of the tests, a systemconfigured in accordance with FIG. 1 was used to obtain results forremoving mercury and hydrochloric acid from an exhaust gas stream of alime kiln. During the test, mercury and hydrochloric acid analyzers werecalibrated, and speciated mercury adsorbent traps were in used in tandemto the online analyzers to verify their accuracy. During the tests, thegas streams were cooled from about 450° F. to between 250-300° F.Effectiveness (%) of the removal of mercury and hydrochloric acid wastested under varying conditions, including using (a) only cooling of thegas stream, (b) cooling of the gas stream and injection ofcalcium-containing particles, and (c) cooling of the gas stream andinjection of PAC.

Hg (Total) Hg²⁺ Hg⁰ HCl Cooling 36% 42% 29% 80% Cooling + Ca-Injection84-86% 79-95% 70-79% 98% Cooling + PAC-Injection 81% 91% 47% 99%

As shown in the table above, cooling of the gas stream alone resulted inremoval of approximately 36% of total mercury, 42% of oxidized mercury,29% of elemental mercury, and 80% of hydrochloric acid. Whencalcium-containing particles were injected into the gas stream alongwith cooling of the gas stream, removal of mercury and hydrochloric acidsignificantly increased to be 84-86% for total mercury, 79-95% foroxidized mercury, 70-79% for elemental mercury, and 98% for hydrochloricacid. For cooling of the gas stream along with PAC injection, removalefficiencies included 81% of total mercury, 91% of oxidized mercury, 47%of elemental mercury, and 99% of hydrochloric acid.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thescope of the invention. Additionally, aspects of the invention describedin the context of particular embodiments or examples may be combined oreliminated in other embodiments. Furthermore, although advantagesassociated with certain embodiments of the invention have been describedin the context of those embodiments, other embodiments may also exhibitsuch advantages. Additionally, not all embodiments need necessarilyexhibit such advantages to fall within the scope of the invention.Accordingly, the invention is not limited except as by the appendedclaims.

Throughout this disclosure, the singular terms “a,” “an,” and “the”include plural referents unless the context clearly indicates otherwise.Similarly, unless the word “or” is expressly limited to mean only asingle item exclusive from the other items in reference to a list of twoor more items, then the use of “or” in such a list is to be interpretedas including (a) any single item in the list, (b) all of the items inthe list, or (c) any combination of the items in the list. Additionally,the terms “comprising,” “including,” “having,” and the like are usedthroughout this disclosure to mean including at least the recitedfeature(s) such that any greater number of the same feature(s) and/orone or more additional types of features are not precluded. As usedherein, the term “about” or “approximately,” when preceding a value(e.g., a temperature value), should be interpreted to mean plus or minus10% of the value, unless otherwise indicated.

I/We claim:
 1. A method for removing one or more air pollutants from agas stream, the method comprising: flowing a gas stream at a firsttemperature toward an exhaust stack, wherein the gas stream includescalcium-containing particles and at least one of mercury (Hg) orhydrochloric acid (HCl); cooling the gas stream to a second temperature,wherein the calcium-containing particles at the second temperature havea higher adsorptive affinity for at least one of the Hg or HCl than thecalcium-containing particles at the first temperature; adsorbing aportion of at least one of the Hg or HCl to the calcium-containingparticles of the gas; and filtering the gas stream to remove at least aportion of the calcium-containing particles having the adsorbed Hg orHCl.
 2. The method of claim 1 wherein the calcium-containing particlesinclude calcium oxide (CaO).
 3. The method of claim 1, furthercomprising injecting calcium hydroxide (Ca(OH)₂) particles into the gasstream.
 4. The method of claim 3, further comprising measuring an amountof at least one of Hg or HCl remaining in the gas stream downstream offiltering, wherein injecting Ca(OH)₂ particles is based at least in parton the measured amount of Hg or HCl.
 5. The method of claim 3 whereinindividual Ca(OH)₂ particles have a surface area of at least about 25m²/g.
 6. The method of claim 1, further comprising: measuring an amountof at least one of Hg or HCl remaining in the gas stream downstream offiltering the gas stream; and adjusting the temperature of the gasstream in response to the measured amount of Hg or HCl, wherein— if themeasured amount of Hg or HCl is above a threshold value, the temperatureof the gas stream is adjusted to a third temperature lower than thesecond temperature.
 7. The method of claim 6, wherein if the measuredamount of Hg or HCl is below a threshold value, the temperature of thegas stream is adjusted to a fourth temperature that is above the secondtemperature.
 8. The method of claim 3 wherein cooling the gas streamincludes cooling the gas stream via a heat exchanger using air as acooling fluid, and wherein injecting Ca(OH)₂ particles includesinjecting Ca(OH)₂ particles downstream of the heat exchanger.
 9. Themethod of claim 8, further comprising, prior to filtering the gasstream, injecting a powder activated carbon (PAC) into the gas stream.10. The method of claim 1 wherein the first temperature is greater thanabout 450° F. and the second temperature is between about 250° F. andabout 325° F.
 11. The method of claim 1 wherein the Hg includes a firstportion of Hg that is elemental mercury having a valence shell that isfull and a second portion of Hg that is oxidized mercury having avalence shell that is not full.
 12. The method of claim 1 wherein thegas stream is lime kiln exhaust originating from a lime kiln facility.13. The method of claim 1 wherein removing the one or more airpollutants from the gas stream does not include injecting powderactivated carbon (PAC) into the gas stream.
 14. The method of claim 1wherein filtering includes filtering the gas stream via a baghouseconfigured to be blown down to remove the adsorbed Hg or HCl from thebaghouse, and wherein the baghouse includes: a multi-compartmentedbaghouse section, a valve arrangement configured to direct the incominggas stream to the multi-compartmented baghouse section, and a controllerconfigured to control actuation of the valve arrangement.
 15. A systemfor removing one or more air pollutants from a gas stream of anindustrial facility, the system comprising: an industrial apparatusconfigured to produce a gas stream having a plurality ofcalcium-containing particles and one or more air pollutants, thepollutants including at least one of mercury (Hg) or hydrochloric acid(HCl), a cooling apparatus downstream of the industrial apparatus andpositioned to receive at least a portion of the gas stream, wherein thecooling apparatus is configured to cool the gas stream to a temperatureless than about 350° F., and wherein the cooled calcium-containingparticles are configured to adsorb a portion of at least one of the Hgor HCl; and a filtering system downstream of the cooling apparatus andpositioned to receive the cooled gas stream, wherein the filteringsystem is configured to remove at least a portion of thecalcium-containing particles having adsorbed Hg or HCl.
 16. The systemof claim 15 wherein the calcium-containing particles comprise a firstplurality of calcium-containing particles, the method furthercomprising: a sensor positioned downstream of the filtering system andconfigured to measure at least one of Hg or HCl; and an injection systempositioned upstream of the filtering system, the injection system beingconfigured to inject a second plurality of calcium-containing particles,based at least in part of the measured Hg or HCl.
 17. The system ofclaim 16 wherein the first plurality of calcium-containing particlesincludes calcium oxide (CaO) and the second plurality ofcalcium-containing particles includes calcium hydroxide (Ca(OH)₂). 18.The system of claim 16, further comprising a controller in communicationwith the sensor, wherein the controller includes instructions that, whenexecuted, cause the injection system to (a) inject more of the secondplurality of calcium-containing particles if the sensed Hg or HCl isabove a first predetermined threshold range, and (b) inject less of thesecond plurality of calcium-containing particles if the sensed Hg or HClis below a second predetermined threshold.
 19. The system of claim 15,further comprising: a sensor positioned downstream of the filteringsystem and configured to measure at least one of Hg or HCl; and acontroller in communication with the sensor and the cooling apparatus,wherein the controller includes instructions that, when executed, causethe cooling apparatus to cool the gas stream based on the measured Hg orHCl.
 20. The system of claim 16 wherein: the industrial apparatus is alime kiln, the calcium-containing particles include lime kiln dust, andthe cooling apparatus includes a heat exchanger using air as a coolingfluid.
 21. The system of claim 16 wherein the calcium-containingparticles and the air pollutants of the gas stream are cooled via thecooling apparatus to a temperature range from about 250° F. to about300° F.
 22. A method for treating a gas stream containing airpollutants, the method comprising: directing a gas stream toward anexhaust stack, the gas stream including a first plurality ofcalcium-containing particles and at least one of mercury (Hg) orhydrochloric acid (HCl), wherein at least a portion of the firstplurality of calcium-containing particles adsorb to at least one of theHg or HCl; injecting a second plurality of calcium-containing particlesinto the gas stream, wherein at least a portion of the second pluralityof calcium-containing particles adsorb to at least one of the Hg or HCl;and filtering from the gas stream (a) at least some of the firstplurality of particles having adsorbed Hg or HCl, and (b) at least someof the second plurality of particles having adsorbed Hg or HCl.
 23. Themethod of claim 22 wherein the first plurality of calcium-containingparticles includes calcium oxide (CaO) and the second plurality ofcalcium-containing particles includes calcium hydroxide (Ca(OH)₂). 24.The method of claim 22 wherein the second plurality ofcalcium-containing particles includes calcium hydroxide (Ca(OH)₂)particles, and wherein individual Ca(OH)₂ particles have a surface areaof at least about 25 m²/g.
 25. The method of claim 22 wherein the secondplurality of calcium-containing particles includes calcium hydroxide(Ca(OH)₂) particles, the method further comprising: measuring an amountof at least one of Hg or HCl remaining in the gas stream downstream offiltering the gas stream, wherein injecting Ca(OH)₂ particles is basedat least in part on the measured amount of Hg or HCl.
 26. The method ofclaim 22, further comprising altering the thermal properties of at leastone of the first plurality of calcium-containing particles or the secondplurality of calcium-containing particles, wherein the first and secondpluralities of calcium-containing particles having the altered thermalproperties have a higher affinity for at least one of Hg or HCl comparedto the first and second pluralities of calcium-containing particleshaving unaltered thermal properties.
 27. The method of claim 22, furthercomprising cooling at least one of the first or second pluralities ofparticles, wherein the cooled first or second pluralities ofcalcium-containing particles have a greater affinity to adsorb Hg and/orHCl relative to the uncooled first or second pluralities ofcalcium-containing particles.
 28. The method of claim 27 wherein coolingincludes cooling to a temperature less than about 300° F.
 29. The methodof claim 22, further comprising, prior to filtering the gas stream,injecting a powder activated carbon (PAC) into the gas stream.
 30. Amethod for treating a gas stream containing air pollutants, the methodcomprising: directing a gas stream at a first temperature toward anexhaust stack, the gas stream including at least one of mercury (Hg) orhydrochloric acid (HCl); cooling the gas stream to a second temperature,wherein the mercury and hydrochloric acid at the second temperature havea greater adsorptive affinity to calcium-containing particles at thesecond temperature than at the first temperature; injecting a pluralityof calcium-containing particles into the gas stream, wherein at least aportion of the plurality of calcium-containing particles adsorb to atleast one of the Hg or HCl; and filtering from the gas stream at leastsome of the plurality of calcium-containing particles having adsorbed Hgor HCl.
 31. The method of claim 30 wherein the calcium-containingparticles comprise calcium oxide (CaO) or calcium hydroxide (Ca(OH)₂).